This invention relates to apparatus for measuring the ratio between gas and liquid in pipes of a nuclear reactor or a boiler through which high temperature and high pressure steam and water flow.
A mixed fluid consisting of a vapor phase fluid as steam and a liquid phase fluid as water is generally termed a two phase fluid, and the ratio of the steam in the two phase fluid is called a void ratio. The void ratio is one of the important items of measurement in a nuclear reactor and a boiler in which a two phase fluid exists.
FIG. 1 shows a prior art void ratio measuring apparatus utilizing radiation. More particularly, a source of X rays 2 and a X ray detector 3 are disposed on the opposite sides of a cylindrical measuring unit 1, and X rays 4 are collimated into parallel beams by a collimeter 5. Then, the X rays transmit through the cylindrical measuring unit 1 to enter into the X ray detector 3 through slits or small openings 6. The output signal from the X ray detector 3 is sent to a signal processing circuit, not shown, via a cable 7. Bubbles of steam entrained in a liquid flowing through the cylindrical measuring unit 1 are designated by reference numerals 8.
In the prior art void ratio measuring apparatus described above, let us denote the output voltages of the detector 3 when the measuring unit 1 is empty and filled with water by I.sub.A volt and I.sub.W volt respectively, and the output voltage of the detector 3 when the two phase fluid to be measured is flowing through the measuring unit 1 by I.sub.X volt. Then the void ratio .alpha. is given by the following equation. ##EQU1## where .rho..sub.W represents the density of water at normal temperature, .rho..sub.W ' that of high temperature water and .rho..sub.V ' the density of steam.
The void ratio .alpha. can be equivalently expressed by an equation ##EQU2## where l.sub.i represents the length of X ray beam 4 transmitting through one of the steam bubbles 8 prevailing in the cylindrical measuring unit 1 (where a plurality of bubbles present the length are shown by l.sub.1, l.sub.2, . . . l.sub.i . . . l.sub.n), and l.sub.D the lengths of the X ray beams 4 transmitting through the measuring unit 1.
Thus, the void ratio measured by the X ray void ratio measuring apparatus is defined by a ratio of the length l.sub.D of the two phase fluid through which the X ray beams transmit to the sum of the lengths of the steam bubbles through which the X ray beams transmit. Such a void ratio is specifically termed a local void ratio.
When the source of X rays 2, the X ray detector 3, the collimeter 5 and the slits 6 are moved in unison in a plane perpendicular to the axis of the cylindrical measuring unit 1, for example, toward the upper end as shown in FIG. 2, a void ratio .alpha.(x) for a height x of the X ray beams can be obtained in the same manner. An average void ratio .alpha. in a cross-section can be obtained by integrating .alpha.(x) from a height of x=-r.sub.0 to a height of x=+r.sub.0 with a height l.sub.D =.sqroot.r.sub.0.sup.2 -x.sup.2 added and then dividing the integrated value with the cross-sectional area A=.pi.r.sub.0.sup.2 of the measuring unit 1. Thus, ##EQU3##
Then, it is possible to obtain an accurate cross-sectional average void ratio .alpha.. According to the prior art void ratio measuring apparatus, as shown in FIG. 3 a single fixed beam was used, or as shown in FIG. 4, three fixed X ray beams have been used. The source of radiation is difficult to handle. In a certain case, a source of .gamma. rays difficult to collimate has been used. With fixed three beams, error of the measurement of the cross-sectional average void ratio often amounts to more than 25% depending upon the mode of flow of the two phase fluid. For these reasons, the prior art apparatus of measuring void ratio with radiations are not suitable for practical use.